Removing an area from a cuboid shape
&#9096; CtrlR  to add a loop cut.
G, G to edge slide.
G to move/grab.
G, Y to move along global Y axis.
&#9095; Alt RMB to select an edge loop.
E to extrude.
&#9096; Ctrl&#8633; Tab to switch the selection context.
B MMB to box deselect.


Double Geometry  (not that good)

Add two loop cuts. Edge Slide the lower loop cuts upwards. Deselect the back vertices and select the lower ones. Move them back along Y. 

N-Gon  (not that good)

Delete the side faces, add two loopcuts and move the vertices together. Move the lower face along Y. Make the missing face F.

Extrusion

Add one loop cut. Select the upper face. Extrude the upper face.  
Multiple meshes

Duplicate the geometry, move and scale it accordingly.




  Now, although you probably had #3 in mind, I'd advise you to think about the benefits of #4. In the real world, these object would be two separate pieces as well, with separate materials.